I have had several fights with renault about my aircon, the first being that it never worked and made a funny noise, they said they would charge me amount to gas to see if there is a problem, my point being is that your stuck with them gassing it - as if you get it done somewhere else (cheaper) they turn round and say we have to do it. Not fair that, after a few phone calls to renault - they gassed the car up - and found that the compressor was faulty - new one put in. same thing a few months later - no gas left, took it back up - they came out with the whole it needs to be used etc or it will let the gas out blah blah. so after explaining it has just had a new compressor and shouldnt let out gas they quickly sorted it out! and touch wood its been fine. Just persist with them!!
As for the warranty - i took it out, came to £330 for me for the year, and i have the invoice and letter stating renault sport - so somehow i got it for the same price as a normal clio. its exactly the same as Renaults warranty - so anything covered under that should be under the extended one.